The prevention and control of viral Infection and disease represents a continuing challenge for both medical and veterinary medicine. As such methods for reducing the incidence and severity of viral diseases remain at the forefront of medical and biotechnological research.
The most common strategy currently utilised for the control of viral infection and disease is a vaccination approach. Vaccines may comprise inactivated viruses or purified products thereof which have treated with chemicals such as an aziridine, for example binary ethyleneimine (BEI). Alternatively vaccines may comprise live, attenuated viruses that have been cultivated under conditions that lead to a loss of virulence whilst retaining immunogenicity. For example, the disease-causing virus may be passaged through a series of cell cultures or animal embryos until a point at which it is sufficiently poor at replicating in its target cells that it can be used as a vaccine. At this point the virus has lost virulence but is still immunogenic and able to stimulate an immune response.
Embryonated chicken eggs and primary chicken embryonic fibroblasts (CEF) are commonly used for the manufacturing of human and veterinary viral vaccines, including traditional large volume vaccines such as influenza or Newcastle Disease vaccines as well as more modern recombinant viral vectors for vaccines (eg. poxviruses). Embryonated chicken eggs and CEFs are used for passaging viruses in order to generated non-virulent attenuated viruses. Examples of vaccines which utilise attenuated viruses include measles vaccine, mumps vaccine, rubella vaccine, oral polio vaccine (Sabin) and yellow fever vaccine.
The global burden of viral diseases on both animal and human health results in a requirement for the mass production of inactivated and attenuated viruses for use in vaccines. In addition, the outbreak of seasonal pandemics, for example of influenza, requires the production of a large amount of virus in a short-time period. In fact, the global availability of a therapeutically effective influenza virus vaccine during a pandemic remains a major challenge for the biopharmaceutical industry.
A current issue impacting the use of embryonated chicken eggs and CEFs in the production of viruses for vaccines is the time takes to generate the large amount of virus that is required. Further, a draw back to the use of live, attenuated viruses in vaccines is the length of time taken to achieve a sufficient number of passages in a host cell in order to generate an attenuated virus. The highly attenuated modified vaccinia virus Ankara (MVA), for example, which serves as a candidate vaccine to immunize against infectious diseases and cancer, required more than 500 passages in CEFs to generate an attenuated MVA which had lost a substantial part of coding genome sequences and demonstrated a severe restriction of replication on mammalian cells. In addition, restrictions on the passage rate of viruses in host cells may serve as a limiting step influencing virus production and yield.
There is thus a need for methods which increase the yield and efficiency of production of both inactivated and attenuated viruses in systems such as embryonated chicken eggs and CEFs for the production of vaccines.